Field effect transistors are devices commonly used in the fabrication of integrated circuitry. Such devices conventionally comprise a pair of conductive source/drain regions having a semiconductive channel region therebetween. A conductive gate is received operably proximate the channel region, and is separated therefrom by dielectric material. Application of suitable voltage to the gate causes current to flow from one of the source/drain regions to the other through the channel region, accordingly operating as a switch depending upon voltage application to the gate.
Transistors used in semiconductor constructions are supported by a semiconductor substrate. Such substrate might comprise bulk monocrystalline substrates, and/or semiconductor-on-insulator substrates. Regardless, the semiconductor substrate will have a primary surface which can be considered to define a horizontal direction. Field effect transistor devices can be divided into two broad categories based upon the orientations of the channel region relative to the primary surface of the semiconductor substrate. Specifically, transistor structures which have channel regions that are generally parallel to the primary surface of the substrate are referred to as planar or horizontal transistor structures. Those having channel regions which are generally perpendicular to the primary surface of the substrate are referred to as vertical transistor structures. Since current flow between the source and drain regions of a transistor device occurs through the channel region, planar transistor devices can be distinguished from vertical transistor devices based upon the direction of current flow as well as on the general orientation of the channel region. Specifically, vertical transistor devices are devices in which the current flow between the source and drain regions is primarily substantially orthogonal to a primary surface of a semiconductor substrate. Planar or horizontal transistor devices are devices in which the current flow between source and drain regions is primarily parallel to the primary surface of the semiconductor substrate.
Memory is one type of integrated circuitry, and is used in computer systems for storing data. Such is typically fabricated in one or more arrays of individual memory cells. The memory cells might be volatile, semi-volatile, or non-volatile. Non-volatile memory cells can store data for extended periods of time, and in many instances including when the computer is turned off. Volatile memory dissipates and therefore is required to be refreshed/rewritten, and in many instances including multiple times per second.
One example volatile semiconductor memory is dynamic random access memory (DRAM). In one example, an individual DRAM cell includes a field effect transistor and a storage capacitor. One of the source/drain regions of the transistor connects with one electrode of the storage capacitor, while the other source/drain region electrically connects with a bit line. In some instances, pairs of immediately adjacent field effect transistors might share a common source/drain region to which a bit line electrically connects. DRAM cells may be characterized as requiring rewriting or refreshing multiple times a second. How often refresh must occur is effected by a number of factors, including degree of current/charge leakage. Two significant components of leakage include channel leakage of the transistor to underlying semiconductor substrate material, and junction or gate induced drain leakage.